The present invention relates to a clock signal generating circuit, and more particularly to a circuit for generating a clock signal used to demodulate an audio signal which is digitized and recorded in multiplex form with other signals such as video signals.
An example of such a system is disclosed in Japanese Patent Application No. 45780/1983. In this system, a video signal and an audio signal are recorded in multiplex form, as a result of which an audio signal of wide dynamic range and high quality can be obtained. In this system, FM signals obtained by subjecting a video signal and an audio signal to frequency modulation and a pulse train signal obtained by digitizing the audio signal according to a predetermined digital modulation system are recorded in multiplex form on a recording medium.
The audio signal contains two channels wherein audio carriers of 2.3 MHz and 2.8 MHz are FM modulated with two respective audio channel signals. The video signal is subjected to frequency conversion in such a manner that the sync peak is 7.6 MHz, the pedestal level is 8.1 MHz, and the white peak is 9.3 MHz. Furthermore, the audio signal is digitized into a pulse train signal according to a modulation system such as a PCM (pulse code modulation) system. The pulse train signal is made suitable for recording, for instance, according to an EFM (eight-to-fourteen modulation) system. The resulting frequency spectrum has frequency components of pulses ranging from 3T to 11T in pulse width (where T is the bit period of the PCM signal, a pulse of 3 T corresponding to about 720 KHz, and a pulse of 11T to about 200 KHz). The pulse train signal is superposed on the video main carrier at a level of less than about one-tenth that of the main carrier can is slice amplified near the zero-crossing point to provide a pulse-width-modulated signal, which is then employed as the recording signal.
FIG. 1 shows the frequency spectrum of a RF (high frequency) signal obtained from a recording medium on which video and audio signals have been recorded according to the above-described recording system. In FIG. 1, reference character A designates a digitized audio signal component; B, an audio FM signal component; C, a color data component in a video FM signal; and D, a luminance data component in the video FM signal.
The dynamic range of the digitized audio signal can be made about 90 dB or more. Therefore, the tone quality upon reproduction of the digital audio signal is considerably high compared with the case where audio signals are recorded and reproduced employing the FM modulation system.
In general, a PLL (phase-locked loop) circuit is employed as a clock signal generating circuit for generating a clock signal to demodulate the digitized audio signal. The PLL circuit includes an oscillator and a phase comparator for subjecting the output of the oscillator and the pulse train signal to phase comparison. The output frequency of the oscillator is controlled by the output of the phase comparator. After the PLL circuit has reached the locked state, in order to maintain the PLL circuit stable, a holding circuit is provided which is adapted to maintain the oscillating frequency of the oscillator stable between two adjacent frame synchronizing signals in the pulse train signal.
The above-described conventional clock signal generating circuit suffers from the following difficulty: If the RF signal is irregular due to scratches or smudges on the recording medium, it takes a period of time (about 130 microseconds) corresponding to at least one frame before the irregularity can be detected. Therefore, in the conventional clock signal generating circuit, the oscillating frequency of the oscillator in the PLL circuit inevitably sometimes becomes irregular. Moreover, it is impossible to correctly demodulate the pulse train signal immediately after the irregularity of the RF signal is eliminated.